1. Field of the Invention
This invention relates to mixers in general and more particularly to a mixer that has high local oscillator to RF (L-R) isolation, low conversion loss and good VSWR at a low cost.
2. Description of Related Art
A mixer circuit converts a radio frequency (RF) signal to an intermediate frequency (IF) signal which is the difference of the RF and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal. The difference or IF frequency is a result of the non-linearity of the mixer. Along with the IF frequency, the mixer typically generates inter-modulation products due to the non-linearity response.
FIG. 1 shows a schematic drawing of a prior art mixer. Mixer 20 has a local oscillator input terminal LO for receiving a local oscillator signal, an RF input terminal RF for receiving an RF signal and an intermediate frequency output terminal IF for providing an intermediate frequency output signal.
A diode ring QD1 has four diodes D1, D2, D3, and D4. The diodes are connected in a ring configuration. The cathode of each diode is connected to the anode of the adjacent diode. Node 22 is connected between diodes D1 and D2. Node 23 is connected between diodes D2 and D3. Node 24 is connected between diodes D3 and D4. Node 25 is connected between diodes D1 and D4.
Mixer 20 has a local oscillator port LO that is connected to local oscillator balun transformer T1. Transformer T1 has windings T1a and T1b that are wound on a core C1. Windings T1a and T1b are magnetically coupled. Winding T1a has one end connected to port LO and the other end connected to ground G. Winding T1b has one end connected to node 22 and the other end connected to node 24. The midpoint of winding T1b is connected to ground. Transformer T2 has windings T2a and T2b that are wound on a core C2. Windings T2a and T2b are magnetically coupled. Winding T2a has one end connected to port RF and the other end connected to ground G. Winding T2b has one end connected to node 23 and the other end connected to node 25. The midpoint of winding T2b is connected to port IF.
The turns ratio of balun transformers T1 and T2 determine the VSWR at the local oscillator (LO) and RF terminals. The amplitude unbalance and phase unbalance of balun transformers T1 and T2 determine the L-R isolation. The insertion loss and matching of diode ring QD1 to balun transformers T1 and T2 determines the conversion loss of the mixer. Due to parasitic capacitance between the primary and secondary windings, the amplitude unbalance of balun transformers T1 and T2 becomes worse at high frequency. The VSWR and insertion loss of balun transformers T1 and T2 becomes worse at high frequency.
While various mixers have been used, they have suffered from not having high local oscillator to RF (L-R) isolation, low conversion loss and good VSWR at a low cost.
A current unmet need exists for a mixer that has high L-R isolation, low conversion loss, good VSWR and that can be assembled at low cost.